Search for CP violation in the decays D+--> K(S)pi+ and D+-->K(S)K+.
ABSTRACT A high-statistics sample of photoproduced charm from the FOCUS experiment has been used to search for direct CP violation in the decay rates for D+-->K(S)pi+ and D+-->K(S)K+. We have measured the following asymmetry parameters relative to D+-->K-pi+pi+: A(CP)(K(S)pi+) = (-1.6+/-1.5+/-0.9)%, A(CP)(K(S)K+) = (+6.9+/-6.0+/-1.5)%, and A(CP)(K(S)K+) = (+7.1+/-6.1+/-1.2)% relative to D+-->K(S)pi+. We have also measured the relative branching ratios and found Gamma(D+-->K(0)pi+)/Gamma(D+-->K-pi+pi+) = (30.60+/-0.46+/-0.32)%, Gamma(D+-->K(0)K+)/Gamma(D+-->K-pi+pi+) = (6.04+/-0.35+/-0.30)%, and Gamma(D+-->K(0)K+)/Gamma(D+-->K(0)pi+) = (19.96+/-1.19+/-0.96)%.
- B. R. Ko, E. Won, I. Adachi, H. Aihara, K. Arinstein, D. M. Asner, T. Aushev, A. M. Bakich, K. Belous, V. Bhardwaj, [......], G. Varner, C. H. Wang, M.-Z. Wang, P. Wang, Y. Watanabe, K. M. Williams, Y. Yamashita, C. C. Zhang, V. Zhilich, A. Zupanc[Show abstract] [Hide abstract]
ABSTRACT: We search for CP violation in the decay ${D^{+}}\to K_S^0{K^{+}}$ using a data sample with an integrated luminosity of 977 fb−1 collected with the Belle detector at the KEKB e +e − asymmetric-energy collider. No CP violation has been observed and the CP asymmetry in ${D^{+}}\to K_S^0{K^{+}}$ decay is measured to be (−0.25 ± 0.28 ± 0.14)%, which is the most sensitive measurement to date. After subtracting CP violation due to ${K^0}-{{\overline{K}}^0}$ mixing, the CP asymmetry in ${D^{+}}\to {{\overline{K}}^0}{K^{+}}$ decay is found to be (+0.08 ± 0.28 ± 0.14)%.Journal of High Energy Physics 02/2013; 2013(2). · 5.62 Impact Factor
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arXiv:hep-ex/0109022v1 17 Sep 2001
Search for CP Violation in the decays D+→ KSπ+and D+→ KSK+
J. M. Link,1M. Reyes,1P. M. Yager,1J. C. Anjos,2I. Bediaga,2C. G¨ obel,2J. Magnin,2A. Massafferri,2
J. M. de Miranda,2I. M. Pepe,2A. C. dos Reis,2S. Carrillo,3E. Casimiro,3A. S´ anchez-Hern´ andez,3C. Uribe,3
F. V´ azquez,3L. Cinquini,4J. P. Cumalat,4B. O’Reilly,4J. E. Ramirez,4E. W. Vaandering,4J. N. Butler,5
H. W. K. Cheung,5I. Gaines,5P. H. Garbincius,5L. A. Garren,5E. Gottschalk,5P. H. Kasper,5A. E. Kreymer,5
R. Kutschke,5S. Bianco,6F. L. Fabbri,6A. Zallo,6C. Cawlfield,7D. Y. Kim,7A. Rahimi,7J. Wiss,7R. Gardner,8
A. Kryemadhi,8Y. S. Chung,9J. S. Kang,9B. R. Ko,9J. W. Kwak,9K. B. Lee,9H. Park,9G. Alimonti,10
M. Boschini,10P. D’Angelo,10M. DiCorato,10P. Dini,10M. Giammarchi,10P. Inzani,10F. Leveraro,10
S. Malvezzi,10D. Menasce,10M. Mezzadri,10L. Milazzo,10L. Moroni,10D. Pedrini,10C. Pontoglio,10F. Prelz,10
M. Rovere,10S. Sala,10T. F. Davenport III,11L. Agostino,12V. Arena,12G. Boca,12G. Bonomi,12G. Gianini,12
G. Liguori,12M. M. Merlo,12D. Pantea,12S. P. Ratti,12C. Riccardi,12I. Segoni,12P. Vitulo,12H. Hernandez,13
A. M. Lopez,13H. Mendez,13L. Mendez,13A. Mirles,13E. Montiel,13D. Olaya,13A. Paris,13J. Quinones,13
C. Rivera,13W. Xiong,13Y. Zhang,13J. R. Wilson,14K. Cho,15T. Handler,15R. Mitchell,15D. Engh,16
M. Hosack,16W. E. Johns,16M. Nehring,16P. D. Sheldon,16K. Stenson,16M. Webster,16and M. Sheaff17
(The FOCUS Collaboration)
1University of California, Davis, CA 95616
2Centro Brasileiro de Pesquisas F´isicas, Rio de Janeiro, RJ, Brasil
3CINVESTAV, 07000 M´ exico City, DF, Mexico
4University of Colorado, Boulder, CO 80309
5Fermi National Accelerator Laboratory, Batavia, IL 60510
6Laboratori Nazionali di Frascati dell’INFN, Frascati, Italy I-00044
7University of Illinois, Urbana-Champaign, IL 61801
8Indiana University, Bloomington, IN 47405
9Korea University, Seoul, Korea 136-701
10INFN and University of Milano, Milano, Italy
11University of North Carolina, Asheville, NC 28804
12Dipartimento di Fisica Nucleare e Teorica and INFN, Pavia, Italy
13University of Puerto Rico, Mayaguez, PR 00681
14University of South Carolina, Columbia, SC 29208
15University of Tennessee, Knoxville, TN 37996
16Vanderbilt University, Nashville, TN 37235
17University of Wisconsin, Madison, WI 53706
(Dated: February 7, 2008)
A high-statistics sample of photo-produced charm from the FOCUS (E831) experiment at Fermi-
lab has been used to search for direct CP violation in the decays D+→ KSπ+and D+→ KSK+.
We have measured the following asymmetry parameters relative to D+→ K−π+π+: ACP(KSπ+) =
(−1.6±1.5±0.9)%, ACP(KSK+) = (+6.9±6.0±1.5)% and ACP(KSK+) = (+7.1±6.1±1.2)% rel-
ative to D+→ KSπ+.The first errors quoted are statistical and the second are systematic.
We have also measured the relative branching ratios and found:
K−π+π+) = (30.60 ± 0.46 ± 0.32)%, Γ(D+→¯ K0K+) / Γ(D+→ K−π+π+) = (6.04 ± 0.35 ±
0.30)% and Γ(D+→¯ K0K+) / Γ(D+→¯ K0π+) = (19.96 ± 1.19 ± 0.96)%.
Γ(D+→ ¯ K0π+) / Γ(D+→
PACS numbers: 11.30.Er 13.20.Fc 14.40.Lb
CP violation occurs when the decay rate of a parti-
cle differs from that of its CP conjugate [1].
Kobayashi-Maskawa ansatz this arises due to the non-
vanishing phase in the Cabibbo-Kobayashi-Maskawama-
trix when the decay amplitude has contributions from
at least two quark diagrams with differing weak phases.
In addition final state interactions (FSI) must provide a
strong phase shift. In the Standard Model direct CP vi-
olation in the charm meson system is predicted to occur
at the level of 10−3or below [2]. The mechanism usu-
ally considered is the interference of the tree and penguin
amplitudes in singly-Cabibbo suppressed (SCS) decays.
In the
In the decay D+→ KSπ+the Cabibbo favored (CF)
and doubly-Cabibbo suppressed (DCS) amplitudes con-
tribute coherently with, perhaps, a different weak phase.1
In addition the isospin content of the DCS amplitude
differs from that of the CF case so we can expect a
non-trivial strong phase shift. Several authors have com-
mented on the effect of K0mixing on the CP asymmetry
for this decay mode and the possibility of using it to
1The charge conjugate state is implied unless stated otherwise.
Page 2
2
search for new physics [3, 4].
Differences in the non-leptonic decay amplitudes of
charmed mesons are almost certainly due to FSI. These
effects tend to be amplified in the charmed system mak-
ing it an ideal laboratory for their study [5]. The isospin
amplitudes and phase shifts in D → KK, D → Kπ and
D → ππ decays can be extracted from measurements of
the branching fractions [6]. For example the magnitude
of the I=3/2 amplitude can be obtained directly from the
D+→¯K0π+partial width [7].
Previous studies of D+→ KSπ+and D+→ KSK+
have concentrated on measuring relative branching ratios
[8, 9]. This paper reports the first measurement of the
CP asymmetry for these decays.
The data were collected during the 1996–1997 fixed
target run at Fermilab. Bremsstrahlung of electrons and
positrons with an endpoint energy of approximately 300
GeV produces a photon beam. These beam photons in-
teract in a segmented beryllium-oxide target and produce
charmed particles. The average photon energy for events
which satisfy our trigger is ≃ 180 GeV. FOCUS uses
an upgraded version of the E687 spectrometer which is
described in detail elsewhere [10]. Charged decay prod-
ucts are momentum analyzed by two oppositely polarized
dipole magnets. Tracking is performed by a system of sil-
icon vertex detectors in the target region and by multi-
wire proportional chambers downstream of the interac-
tion. Particle identification is performed by three thresh-
oldˇCerenkov counters, two electromagnetic calorimeters,
an hadronic calorimeter, and by a system of muon detec-
tors.
The D+→ K−π+π+decay is reconstructed using a
candidate driven vertexing algorithm. A decay vertex
is formed using the reconstructed tracks after which the
momentum vector of the parent D meson is intersected
with other tracks in the event to form a production ver-
tex. The confidence level of the secondary vertex is re-
quired to be greater than 1%. The likelihood for each
charged particle to be an electron, pion, kaon or proton
based on the light yield from each thresholdˇCerenkov
counter is computed [11]. We demand that the Kaon
hypothesis WK, (i.e. −2ln(kaon likelihood)), be favored
over the pion hypothesis Wπ by ∆W = Wπ− WK ≥
1.We also make a pion consistency cut by finding
the alternative minimum hypothesis Wminand requiring
Wmin− Wπ > −2 for both pions. We eliminate con-
tamination due to D∗+→ D0(→ K−π+)π+by asking
that neither Kπ invariant mass combination lies within
25 MeV/c2of the nominal D0mass.
The techniques used for KS reconstruction are de-
scribed elsewhere [12]. Because 90% of KSdecays occur
after the KS has passed through the silicon strip detec-
tor we are unable to employ the same vertexing algorithm
used to reconstruct the D+→ K−π+π+decay. Instead
we use the momentum information from the KS decay
and the silicon track of the charged daughter to form a
FIG. 1: Invariant mass plots for D+→ KSπ+and
D−→ KSπ−
candidate D vector. This vector is intersected with candi-
date production vertices which are formed from two other
silicon tracks. As a final check we force the D vector to
originate at our production vertex and calculate the con-
fidence level that it verticizes with the charged daughter.
This confidence level must be greater than 2%. We re-
quire that the momentum of the charged daughter be
greater than 10 GeV/c, that the confidence level for it to
be a muon be less than 1%, and that it traverse the entire
length of the spectrometer. For the decay D+→ KSπ+
we demand that Wmin−Wπ> −6 and Wπ−WK< 0, for
D+→ KSK+we ask that the kaon hypothesis be favored
over both the proton and pion hypotheses by requiring
Wp− WK> 0 and Wπ− WK> 3. We remove electron
contamination by ensuring that the charged D daughter
links to only one silicon microstrip track. Electron pairs
usually have a very small opening angle in the silicon and
chamber tracks tend to link to both tracks. Checks for
electron contamination of the KSsample using the elec-
tromagnetic calorimeters showed no significant effect. We
use only KS candidates which have a normalized mass2
within three standard deviations of the nominal value.
Additionally, to reduce backgrounds in the D+→ KSK+
mode, we do not use the category of KSdecays which oc-
cur downstream of the silicon where both KSdaughters
lie outside the acceptance of the downstream magnet. We
make the same cut on the D+→ KSπ+normalization
2The normalized mass is the difference between the measured and
the nominal mass divided by the error on the measured mass.
Page 3
3
FIG. 2: Invariant mass plot for D+→ KSK+and
D−→ KSK−. The shaded area is the smoothed background
shape from D+
s → K∗+¯ K0and D+
s →¯ K∗0K+.
signal. When the KS decays in the silicon detector we
demand that all three tracks be inconsistent with origi-
nating at the same vertex. This eliminates backgrounds
from decays such as D+→ π−π+π+.
For all modes we require that the production vertex
have a confidence level greater than 1%, that the maxi-
mum confidence level for a candidate-D daughter track
to form a vertex with tracks from the primary vertex
be less than 20%, that the significance of separation of
the production and decay vertices be greater than 7.5
and that both vertices lie upstream of the first trigger
counter. The momentum of the D must be greater than
40 GeV/c. In Figures 1, 2 and 3 we show the invari-
ant mass distributions for the decays KSπ+, KSK+and
K−π+π+respectively.
We construct the CP asymmetry, ACP as the difference
in the yields, (corrected for efficiency and acceptance),
of the decay in question divided by the sum. We must
also account for differences in production between the
D+and D−. To do this we ratio the corrected yields to
those of a Cabibbo favored decay which is assumed to be
CP conserving. We measure:
ACP=η(D+) − η(D−)
η(D+) + η(D−)
where for example,
η(D+) =
N(D+→ KSπ+)
N(D+→ K−π+π+)
is the ratio of the corrected yields for each decay which
is equivalent to the relative branching ratio.
FIG. 3: Invariant mass plot for D+→ K−π+π+and
D−→ K+π−π−. The shaded area is the third-degree
polynomial background in the fit region.
To account for non-Gaussian tails in the D+
K−π+π+signals we find it necessary to fit these distribu-
tions using two Gaussians and a third-degree polynomial.
The KSπ+distribution is fit using a Gaussian and a
linear polynomial. The non-linear background shape be-
low 1.75 GeV/c2in the KSπ+plot is primarily due to
D+→ KSl+νland is not included in the fit.
We fit the KSK+signal using a combination of a
Gaussian, linear polynomial, and a background shape
derived from Monte Carlo. This shape is a smoothed
fit to D+
due to a missing π0, are responsible for the background
shape below the D+peak. Because of the difficulty in
fitting the region between the D+and D+
KSK+distribution we only fit up to 1.935 GeV/c2. To
minimize systematic errors we change the KS selection
cuts on the D+→ KSπ+normalization signal to match
those used for the D+→ KSK+mode. The yield for the
decay D+→ KSπ+changes to 4487±96 events and for
D−→ KSπ−becomes 4770±96 events. We can now cal-
culate the relative branching ratios and CP asymmetries.
The results are shown in Tables I and II.
We studied systematic effects due to uncertainties in
our Monte Carlo production model, reconstruction al-
gorithm, and variations in our selection cuts. For the
D+→ KSπ+measurements we split the sample into
eight statistically independent subsamples based on D+
momentum, loose and tight normalized KS mass cuts,
and the time period in which the data were collected.
The momentum dependence of the result arises mainly
due to uncertainties in the parameters used to generate
→
s→ K∗+¯K0and D+
s→¯K∗0K+decays which,
speak in the
Page 4
4
TABLE I: Relative branching ratio results. The first error
is statistical and the second is systematic. We account for
the decay chain ¯ K0→ KS → π+π−by multiplying our
KS numbers by a factor of 2.91 assuming that
¯ K0π+) = 2 × Γ(D+→ KSπ+) ; we then quote these results
in terms of¯ K0.
MeasurementResult
Γ(D+→¯
K0π+)
Γ(D+→K−π+π+)(30.60 ± 0.46 ± 0.32)%
Γ(D+→¯
K0K+)
Γ(D+→K−π+π+)
(6.04 ± 0.35 ± 0.30)%
Γ(D+→¯
K0K+)
Γ(D+→¯
Γ(D+→
PDG Average [13]
(32.0 ± 4.0)%
(7.7 ± 2.2)%a
K0π+)(19.96 ± 1.19 ± 0.96)%(26.3 ± 3.5)%
aThis is the measurement of reference 6 with statistical and sys-
tematic errors added in quadrature.
TABLE II: CP asymmetry measurements. The first error is
statistical and the second is systematic.
Measurement
ACP(KSπ+) w.r.t. D+→ K−π+π+
ACP(KSK+) w.r.t. D+→ K−π+π+(+6.9 ± 6.0 ± 1.5)%
ACP(KSK+) w.r.t. D+→ KSπ+
Result
(−1.6 ± 1.5 ± 0.9)%
(+7.1 ± 6.1 ± 1.2)%
our Monte Carlo. The D+→ KSπ+topology and recon-
struction algorithm is substantially different from that of
the D+→ K−π+π+and the two modes differ in how
well the Monte Carlo matches to the data. For example
there is a slight difference in how well the generated and
accepted momentum distributions agree in each case. We
use a technique modeled after the S-factor method used
by the Particle Data Group [13] to evaluate the system-
atic error. A scaled variance is calculated using the eight
independent subsamples. The split sample systematic is
defined as the difference between the scaled variance and
the statistical variance when the former exceeds the lat-
ter. Due to the smaller statistics in the D+→ KSK+
decay mode we can only form four independent subsam-
ples. These are based on the run period in which the
data were collected and on the normalized KSmass.
We evaluate systematic uncertainties due to the fitting
procedure by calculating our results for various fit con-
ditions, such as rebinning the histograms, changing the
background shapes and in the case of D+→ KSK+also
fitting the Dspeak. Since these different results are all a
priori likely we use the resulting sample variance as a sys-
tematic. The total systematic is calculated by adding the
fit-variant systematic and the split-sample systematic in
quadrature. For the D+→ KSπ+measurements the sys-
tematic has contributions from both the split-sample and
fit-variant analyses. For the Γ(D+→¯K0π+) / Γ(D+→
K−π+π+) measurement the contribution from the split-
sample is 0.301% and from the fit-variant 0.098%. For the
ACP(KSπ+) measurement the split-sample contribution
is 0.92% and that of the fit-variant is 0.13%. We find no
systematic contribution to the D+→ KSK+measure-
ments from the split-sample technique, and therefore the
fit-variant contributions are identical to the total system-
atic error and are as shown in Tables I and II. Due to the
lower statistics we did not split the D+→ KSK+sample
by momentum. Instead we treat the weighted average of
two samples split by momentum as a fit variant.
To conclude, we have searched for evidence of direct
CP violation in the decays D+→ KSπ+and D+→
KSK+and measured their branching ratios relative to
each other and to D+→ K−π+π+. Our relative branch-
ing ratios are a considerable improvement over previous
measurements. The CP asymmetries have not been pre-
viously measured for these modes and are consistent with
zero.
We wish to acknowledge the assistance of the staffs
of Fermi National Accelerator Laboratory, the INFN of
Italy and the physics departments of the collaborating
institutions. This research was supported in part by the
National Science Foundation, the U.S. Department of En-
ergy, the Italian Istituto Nazionale di Fisica Nucleare and
Minsitero dell’Universit` a e della Ricerca Scientifica e Tec-
nologica, the Brazilian Conselho Nacional de Desenvolvi-
mento Cient´ ıfico e Tecnol´ ogico, CONACyT-M´ exico, the
Korean Ministry of Education and the Korean Science
and Engineering Foundation.
[1] I. Bigi and A. Sanda, CP Violation (Cambridge Uni-
versity Press, The Edinburgh Building, Cambridge CB2
2RU, UK, 2000).
[2] F. Buccella, M. Lusignoli, G. Miele, A. Pugliese, and
P. Santorelli,Phys. Rev. D51, 3478 (1995),
ph/9411286.
[3] I. I. Bigi and H. Yamamoto, Phys. Lett. B349, 363
(1995), hep-ph/9502238.
[4] H. J. Lipkin and Z.-Z. Xing, Phys. Lett. B450, 405
(1999), hep-ph/9901329.
[5] J. L. Rosner, Phys. Rev. D60, 114026 (1999), hep-
ph/9905366.
[6] M. Bishai et al. (CLEO), Phys. Rev. Lett. 78, 3261
(1997), hep-ex/9701008.
[7] M. Bauer, B. Stech, and M. Wirbel, Z. Phys. C34, 103
(1987).
[8] J. C. Anjos et al., Phys. Rev. D41, 2705 (1990).
[9] P. L. Frabetti et al. (E687), Phys. Lett. B346, 199
(1995).
[10] P. L. Frabetti et al. (E-687), Nucl. Instrum. Meth. A320,
519 (1992).
[11] J. M. Link et al. (FOCUS), accepted for publication in
NIM-A, FERMILAB-Pub-01/243-E, hep-ex/0108011.
[12] J. M. Link et al. (FOCUS), submitted to NIM-A,
FERMILAB-Pub-01/244-E.
[13] D. E. Groom et al. (Particle Data Group), Eur. Phys. J.
C15, 1 (2000).
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